1. Field of the Invention
The present invention concerns a stereolithography method for the production of a three-dimensional object, as well as a stereolithography machine using said method.
2. Present State of the Art
As is known, a stereolithography machine comprises a container for a liquid substance suited to be solidified through exposure to predefined radiation, typically light radiation.
The above mentioned radiation is produced by radiation emitting means suited to selectively irradiate a layer of the liquid substance having a predefined thickness and arranged adjacent to the bottom of the container, so as to solidify it.
The machine also comprises a modelling plate facing the bottom of the container and provided with a supporting surface for the three-dimensional object to be made.
The above mentioned modelling plate is associated with moving means that are suited to move it according to a direction perpendicular to the bottom of the container.
In order to produce a three-dimensional object using a machine of the type described above, the shape of the object is schematized as a sequence of layers having a predefined thickness.
According to the production method, the modelling plate is arranged with the supporting surface immersed in the liquid substance, at a distance from the bottom of the container that is equal to the thickness of the first layer of the object.
A layer of liquid substance is thus formed that is adjacent to the bottom of the container and that is selectively irradiated by the emitting means in the parts corresponding to the surface area of the first layer, so as to form a corresponding solidified layer that adheres to the supporting surface of the modelling plate.
Successively, the modelling plate is first moved away from the bottom of the container, so as to separate the solidified layer from the bottom itself and thus allow the liquid substance to flow back under the modelling plate and the liquid layer necessary to form a successive layer of the object to be consequently restored.
Successively, the modelling plate is moved near the bottom of the container, so as to arrange it at a distance from the bottom that corresponds to its distance as it was during the formation of the first layer, increased by the thickness of the successive layer.
The new layer of the object is thus formed analogously to the previous one and this process is repeated until all the layers forming the object have been produced.
Document US 2010/0262272 discloses a method similar to the one described above, except that the layers are formed and solidified between the modeling plate and a solidification substrate located above it, instead of using the bottom of the container.
The method described above poses the drawback that the detachment of the solidified layer from the bottom of the container while the plate is being moved away generates a certain resistance.
This resistance to detachment is mainly due to the suction effect caused by the contact between the solidified layer and the bottom of the container and partly to the adhesion of the solidified layer to the bottom of the container.
Said resistance to detachment produces traction forces on the three-dimensional object being formed and on the bottom of the container, whose amount mainly depends on the speed with which the plate is moved away, on the surface area of the solidified layer and on the physical properties of the liquid substance.
Due to the above mentioned traction forces, it is necessary to limit the said speed of the modelling plate, in order to avoid breaking the three-dimensional object that is being formed.
Consequently, another drawback lies in that the time necessary for the formation of each layer increases, thus increasing also the total time necessary for the production of the object.
A further drawback lies in that the traction forces generate fatigue stress on the bottom of the container, which over time causes the latter to break down.
This involves the need to periodically replace the container, with the inconvenience of having to stop production and bear the replacement costs.
According to a known stereolithography method adopted in the attempt to limit the resistance to detachment described above, the movement of the plate away from the bottom of the container is controlled in such a way that the traction forces acting on the object and on the bottom are limited to a maximum predefined value.
According to the above mentioned method, it is necessary to determine the traction forces, which requires the use of a suitable sensor, and this makes the stereolithography machine more complex and increases its costs.
According to a variant application of the above mentioned method, the traction forces are determined using a numerical calculation procedure.
Even if this variant makes it possible to avoid using a sensor, it however poses the drawback of requiring a complex processing software for calculating the forces.
Furthermore, said calculation may not correspond to the actual value of the forces, with the inconvenience of reducing the reliability of the system.